Pluto is presently classified a dwarf planet instead of a regular planet. For a long time (~75 years) is was considered the 9th planet in the solar system but in 2006 the official definition of planet was changed and the new definition excluded Pluto. This is due in no small part to the discovery of Eris (another dwarf planet) that also orbits the sun but is more massive than Pluto. So rather than add a tenth planet, the definition was changed. I still think Pluto should be considered a planet because it has a moon (Charon) orbiting it.